There he aligned with the ultra-left group led by Alexander Bogdanov, who challenged Lenin for the leadership of the Bolsheviks, and worked on the newspaper Vpered (Forward).

After the outbreak of war in 1914, he worked on the newspaper Nashe Slovo and acted as the main contact between the Bolsheviks and the smaller group associated with Leon Trotsky.

During the Russian Civil War, Manuilsky worked in the People's Commissariat for Food, before being sent to Ukraine, where Lenin assigned him the task of organising the peasant population around Kharkiv to defeat the White Army of Anton Denikin.

[2] In 1926, he supplanted Nikolai Bukharin as leader of the Soviet Union delegation on Comintern's executive, and the lead representative at congresses of the French, German, and Czechoslovak communist parties.

"[4] The Montenegrin communist Milovan Djilas, who met Manuilsky in 1944, admired his learning and writing talent, but remembered him as "a slight and already hunched veteran, dark-haired, with a clipped moustache [who] spoke with a lisp, almost gently and – what astonished me at the time – without much energy."